IE div width + scrollbar

I have a modal UI in my web app that necessitates a fixed layout design. After much fussing, I decided absolutely positioned child elements was the way to go. However, I have one div that conditionally requ开发者_开发技巧ires a vertical scrollbar (if there's too much content). When this scrollbar is added, the width of the div seems to vary between IE 7 and IE 8.

IE8 standards mode:

IE7 standards mode:

I haven't even tested for IE 6, but I'm sure it's even funkier. What's the simplest, least painful way to address this to arrive at a consistent UI in all major browsers?

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Why not just float the thumbnails left instead of absolute positioning? It would be much more consistent over browsers. Than wrap it all in a div with a fixed height and set it's overflow to auto.